The invention relates to methods and apparatus for the control of tape turnaround in video, particularly color video, tape recording systems.
In one of its aspects the invention may be regarded as an improvement of the turnaround control arrangement disclosed in U.S. Pat. No. 3,958,272 which issued to Rotter et al. on May 18, 1976. The arrangement disclosed in this reference provides for the recording, at the moment when the reversing operation of the tape transport is to be initiated, of a high frequency signal in a range, say 5 to 5.5 mhz, lying above the highest frequencies of the modulated video signal; in playback these high frequency signals are detected as a dropout in a predetermined frequency range and a reversal initiating control signal is produced if the duration of the dropout signal is found to be greater than a predetermined time interval.
While the technique disclosed in the reference, generally speaking, results in a highly accurate control of the reversal of tape movement, it is subject to the danger that at the high carrier frequencies mentioned, dropouts due to dust or abrasion particles and the like between the magnetic head and the tape may on occassion cause turnaround to be triggered at incorrect times. The reason for this is that the signal loss in decibels due to dropout is directly proportional to the frequency, or indirectly proportional to the wave length. More particularly, the loss in db is equal to 55 .times. d/.lambda., where d is the separation of head and tape and .lambda. is the wave length. Thus at 5 mhz a dust particle of a given size may give rise to a 20 db loss for example. However, the same dust particle would produce only a 2 db loss at, say, 500 khz. Otherwise expressed, at 500 khz it would take a particle about ten times as large to produce a dropout of the order of 20 db and particles of this size are much less likely to occur.